This invention pertains to flywheel energy storage systems and more particularly to a stationary flywheel storage device that employs a five active axes magnetic bearing system to support a solid alloy steel flywheel for rotation about a vertical axis. The combination and attributes of the particular elements in the system allow for simpler operation, reduced costs and increased life and reliability.
Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems and electric vehicles.
Modern flywheel energy storage systems convert back and forth between the rotational inertia of a spinning flywheel and electrical energy. A flywheel energy storage system includes a flywheel, a motor generator, a bearing system and a vacuum enclosure. The rotating flywheel stores mechanical energy, the motor generator converts between electrical energy and mechanical energy and the bearing system physically supports the rotating flywheel.
In almost all energy storage applications, whether quick discharge type (power ride-through), where discharge time is measured in seconds, or long-term discharge type (power backup), where discharge time is measured in hours, flywheels directly compete with electrochemical batteries. Key advantages of flywheels used for electrical energy storage over electrochemical battery systems are its longevity and reliability, and its lower life cycle cost. Electrochemical batteries, in particular, lead-acid batteries, have short lifetimes, between six months and seven years depending on operating conditions. These batteries require periodic maintenance and can fail unpredictably. In contrast, flywheel energy storage systems are expected to have maintenance-free lifetimes of twenty years.
Magnetic bearings have been used in some flywheel systems for support of the flywheel. They offer the advantages of low friction, no wear, and potential for extremely long life at high operational speeds. One type of magnetic bearing system uses five actively controlled axes to support a rotor. Five active axes magnetic bearings use electromagnets, position feedback and electronic control to provide levitation. Use of such systems is growing in many applications because they allow support and smooth rotation of dynamically complex rotor systems. They also typically can provide much higher radial stiffnesses and carry heavier loads than much simpler passive radial magnetic bearings. They also do not require the large amount of relatively expensive magnets required by passive radial magnetic bearings.
Flywheel systems must operate continuously at high speed for long periods of time. Although magnetic bearings do not encounter wear like mechanical bearings, five active axes magnetic bearing systems require constant high frequency power switching to actively control the radial magnetic bearings. This switching limits the life of the electronics, which eventually fail. To date, some flywheel systems have been built using five active axes magnetic bearings. Unfortunately, these flywheel systems have been extremely complex, costly and have limited reliability. For these reasons, flywheel systems employing this type of bearing are not believed to be commercially available or feasible.
Accordingly, the invention is a stationary flywheel storage device that employs a five active axes magnetic bearing system. The flywheel system is specifically designed to promote longer life, simpler operation, higher reliability and less costly active magnetic bearing support. The flywheel system is intended for use in stationary applications where it is not subjected to the dynamic loads of mobile applications and as such the loads on the magnetic bearings are greatly reduced. The load capacity of the magnetic bearings directly affects the cost of the actuators and amplifier electronics. The actual level of carried loads also reduces the life of the electronics by requiring switching of higher currents. It is therefore a goal of the invention to reduce the loads to the radial magnetic bearings as much as possible for the benefits of both reducing the costs and increasing the life of the electronics.
The loads to the radial magnetic bearings are further reduced by employing rotation about a vertical axis. This is typically required when passive radial magnetic bearings are employed but not necessarily a requirement when active radial magnetic bearings are used. The five active axis magnetic bearing system uses an upper radial magnetic bearing a lower radial magnetic bearing and at least one axial thrust magnetic bearing. The lower the tilt angle from vertical, the lower the radial loads that must be carried and hence the lower the currents that must be switched to provide support. In one embodiment of the invention, the tilt angle from vertical is preferably maintained less than 10 degrees. The radial load capacity of the radial magnetic bearings can be significantly reduced to reduce the cost of the actuators and control amplifiers. The radial magnetic bearings in the assembled flywheel system preferably have load capacities within a range corresponding to a tilt angle between 2 degrees and 10 degrees. Below 2 degrees, installation accuracy becomes more difficult and above 10 degrees, the magnet bearings become more expensive.
Unlike previous flywheel systems designed for employing five active axes magnetic bearing systems. The devices according to this invention does not use a composite flywheel. Composite material filament wound flywheels can operate at very high tip speeds for storing large amounts of energy per weight. They are constructed in the form of predominantly hoop wound rims. The rim is then typically connected to a central shaft through the use of a hub that is sufficiently flexible to match the large inner diameter strain of the rim. This construction typically results in flexural resonance modes in the flywheel below the maximum normal operating speed. Five active axes magnetic bearing systems are especially well suited to support these flywheels because the electronic control algorithms can be made to change the stiffnesses and damping in the bearings during operation to provide smooth rotation. Unfortunately, this makes the magnetic bearing control much more complex and it is the belief of this inventor that it is inherently less reliable. If the properties of the flywheel or the electronics change and degrade slightly over the continuous operating life, the magnetic bearings would fail to operate properly. The invention overcomes these problems by employing a steel flywheel having a solid center. The flywheel is preferably constructed from alloy steel and is thus capable of operation above 200 m/sec for storing appreciable energy. The solid center construction serves two purposes: it reduces the hoop direction stresses in the flywheel by 50%, and it also provides a flywheel that is rigidly constructed. In a further aspect of the invention, the flywheel preferably has no flexural resonances below the normal operating speed. The magnetic bearing control can therefore be made much simpler and more reliable. Speed independent control can be used to control the magnetic bearings.
The use of a solid steel flywheel can create some difficulties in the design of the magnetic, bearing system. In composite material flywheels, the stress is maximum in the hoop wound rim and the shaft in the center of the flywheel typically operates at a very low stress level. In a solid steel flywheel, the hoop and radial stresses are maximum in the radial center. When operating at high speeds, these stresses are very large. The result of these stresses is that the length of the flywheel in its radial center will shrink due to Poisson""s ratio contraction. Therefore, the distance between the rotor portions of the upper radial magnetic bearing and the lower radial will shrink. The coupling between the rotor and stator portions of the radial magnetic bearings and bearing properties will change as the speed increases. The amount of contraction is a function of the axial length of the flywheel and also the operating stresses, which are a function of the tip speed. In one embodiment of the invention, the radial magnetic bearings are preferably designed to change the operative axial length by only 5% when rotated to high speed. If the axial position of the flywheel is fixed on the end containing the active axial magnetic bearing or axial position sensor, only the radial magnetic bearing at the opposite end will be affected by the flywheel length change. The radial magnetic bearing on the end of the flywheel opposite the end with the active axial magnetic bearing is therefore designed to have a minimum operative axial length that depends on the axial length of the flywheel and the operating tip speed. Satisfying this requirement also helps the flywheel system overcome any dimensional changes for thermal expansions or contractions.
The magnetic bearing system comprises at least 5 amplifiers with one for each of the five controlled axes and at least one controller. Because the flywheel system is intended to provide power when power is not available, the magnetic bearing system must also derive its own power from the flywheel system. The controller and five amplifiers are preferably powered by one or more DC-DC converters that are connected to the direct current side of a synchronous inverter that drives the motor/generator of the flywheel system. The converters provide constant power regardless of the slowing speed of the flywheel during a discharge.
In all embodiments of the invention, the magnetic bearings are preferably homopolar in that the magnetic fields do not alternate polarity around a given circumferential location. This significantly increases efficiency, reduces heating in the evacuated flywheel environment and reduces power requirements of the electronics. The magnetic bearings are also preferably permanent magnet biased. Permanent magnets provide bias flux in the magnetic bearings which produces several benefits. The bias flux linearizes and amplifies the response of the magnetic bearings for much easier and simpler control. Compared with designs using electromagnetic bias, permanent magnet bias results in lower-power consumption and increased linearity in force to displacement response due to the large reluctance offered by the permanent magnets. Permanent magnet bias also allows use of only one amplifier per axes instead of two. This greatly reduces the costs and increases reliability.